Hi i don't know exactly when they go on sale. I worried about getting tickets last year , and ordered them through the festival website only to find they were easy enough to buy from the ticket office in the village , and even on each day of the festival , although it is i think cheaper to buy them for the full four days . If you haven't been before as was the case with me i found there was a festival within a festival with some fantastic guitarists just jamming on the camp sites i was within a few feet of Stochelo Rosenberg at a wedding on one of the camp sites it turned out to be one of the best couple of hours of the whole week for me , hope the info helps
Dave